EDGE (enhanced data rates for global evolution) further evolution candidates have been presented in GERAN (GSM (global system for mobile communications)/EDGE radio access network) 3GPP (3d generation partnership project). Dual Symbol Rate (DSR) for uplink performance improvement is proposed. As shown in 3GPP contributions, e.g., in GP-052610, Agenda Item 7.1.5.5, “Updates for Dual Symbol Rate Section of the Feasibility Study on Future GERAN Evolution”, 3GPP TSG GERAN#27, Atlanta, USA. In the DSR, the symbol rate of the GSM/EDGE is doubled and the transmitter signal is allowed to overlap adjacent carriers. The DSR nearly doubles UL (uplink) data spectral efficiency and is, therefore, the interesting UL capacity enhancement feature for the EDGE evolution. From the system performance point of view, frequency planning needs to be considered carefully because adjacent DSR carriers are partially overlapping, which “brakes” the basic frequency planning that is made for the normal 200 kHz carriers because the DSR carriers have a spectrum of approximately 600 kHz wide compared to the normal 200 kHz wide carriers as shown in FIG. 1. In the DSR concept the symbol rate was doubled, thus doubling the bit rate over the air interface can be obtained with the same modulation. This makes it possible to use the current EGPRS (enhanced general packet radio service) coding schemes for the DSR, only transmit them with the double bit rate. Thus, the original EGPRS link adaptation and incremental redundancy are compatible with the DSR.
Also in the case of EGPRS, interference conditions need to be considered when data connections are allocated to the hopping layer. Data connections are typically causing more interference than speech connections (e.g., because data uses higher transmitter powers since C/I (carrier-to-interference ratio) and the target is higher compared to AMR/FS (adaptive multi-rate full rate speech).
As shown in FIG. 1, the DSR carrier overlaps with adjacent carriers so that the interference situation is worse in the network using DSR; then the original frequency reuse is blurred in the DSR case. As adjacent DSR carriers are overlapping, usage of DSR makes the interference situation uncontrolled when basic frequency planning is used.
Moreover, in the case of the EGPRS, increased interference from data connections can be a problem, data traffic is allocated to hopping layer which was originally planned for the speech traffic only. Increased interference decreases speech traffic performance.
In the GSM system, co-channel and adjacent channel interference is controlled with the frequency planning. Data and speech traffic can be separated for different frequencies so that speech and data are not interfering with each other. Data traffic can be allocated to BCCH (broadcast control channel) frequencies as far as there are enough resources in a BCCH TRX (transceiver). But, when the BCCH TRX capacity is not enough for the data transmission, a certain amount of hopping layer resources need to be reserved for data. In that case, speech and data connections are interfering with each other. The EGPRS power control is one way to control the interference caused by the data traffic, but then the trade-off between the data throughput and the speech quality is made.
For the DSR concept proposed for the EDGE evolution in 3GPP there are no specific solutions available to control interference caused by wider DSR carriers. As stated in the DSR feasibility study (see GP-052610 quoted above), the current solution is to use IRC (interference rejection combining) receivers and try to cope with increased interference in the network. Also, advanced channel allocation methods which allocate channels based on interference conditions could be used, like proposed in the invention “Radio channel allocation and link adaptation in cellular telecommunication system” by Jari Hulkkonen and Olli Piirainen, filed as a Finnish patent application No. 20055687 on Dec. 21, 2005, but those require more complex allocation algorithms, interference evaluation, etc.
Furthermore, the DSR requires a large bandwidth (3 dB bandwidth is 541 kHz) from the BTS receiver and it seems to cause some possible problems in some infra vendors (i.e., other BTS manufacturers), e.g., with a frequency step size of frequency synthesizers, sample rate for analog-to-digital converters and/or analog filtering. (It should be noted that the evolution items should preferably include only software changes) in the infra side.
Currently, the activity for developing continuation for the existing EDGE standard, which are agreed upon, include mainly downlink improvements (such as spatial diversity and dual carrier). In order to really be able to improve the coverage of the system and improve the data rates, the uplink should be considered more carefully. Current GSM/EDGE algorithm developments contain high performance IRC algorithms and the impacts of interference to radio link performance can be effectively mitigated. Still, in the existing GSM/EDGE networks the capabilities of IRC algorithms have not been fully utilized.